a) Field of the Invention
The invention relates to an arrangement for the simultaneous optical determination of absolute physical state variables and relative changes in physical quantities in an optical fiber, in particular, for determining the temperature and strain fluctuations in an optical fiber for the purpose of precisely determining the changes in environmental influences and/or fiber loads. The invention is preferably applied in processes in which there is a desire or need for current detection of random fluctuations in temperature and pressure, or of fiber stresses.
b) Description of the Relevant Art
It has long been known to use optical fibers and fiber gratings to measure temperature or strain (Fiber optic and laser sensors VII, Proc. of SPIE, Vol. 1169 (1989), pages 98-107), use being made of suitable optical spectral analyzers (edge filters, grating spectrometers, Fabry-Perot etalons), which make absolute measurements, to determine the Bragg wavelength and the value, resulting therefrom by calculation or calibration, of the state variable. Customary limits for the resolution and accuracy of these methods are approximately 1 to 10 pm for the Bragg wavelength and thereby approximately 0.1 to 1 K for the temperature and 0.5 to 5 .mu..epsilon. for the relative strain. A plurality of optical fiber gratings along an optical fiber can be evaluated simultaneously by designing them with their Bragg wavelengths differing greatly (by a few nm), with the result that these reflections can be measured spectrally independently of one another. These methods have become known through A. D. KERSEY et al. ("Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter", in: Optics Letters 18 (1993) 16, pages 1370-1372) as wavelength division multiplexing of the individual sensors and subsequent demultiplexing of the measurement signals.
Furthermore, it is known from Electronic Letters 28 (1992) 3, pages 236-238 (A. D. KERSEY et al.: "High resolution fiber grating based strain sensor with interferometric wavelength-shift detection") to determine strain fluctuations by precise wavelength resolution where the light is fed into an interferometer with asymmetric arm lengths, and it is possible to deduce the temporal characteristic of the change in the state variable from the temporal characteristic of the resulting optical phase difference at the interferometer output. Customary resolution limits are then 1 to 10 n.epsilon. for the amplitude of a strain fluctuation in the frequency band above 10 Hz.
According to a publication by T. A. BERKOFF et al. (in: IEEE Photonics Technology Letters 8 (1996) 11, pages 1522-1525, "Fiber Bragg grating array sensor system using a bandpass wavelength division multiplexer and interferometric detection") for the purpose of simultaneously evaluating a plurality of optical fiber gratings wavelength filters were connected downstream of the interferometer in order to determine the optical phase of the various wavelength components of the various optical fiber gratings separately.